This invention relates generally to a cardiac rhythm management device such as implantable cardiac pacemakers and implantable cardiac defibrillators. In particular the invention pertains to methods and apparatus for automatically adjusting the sensing threshold of such devices.
Currently available implantable cardiac rhythm management devices, including bradycardia and tachycardia pacemakers and cardiac defibrillators, have sense amplifier circuits for amplifying and filtering electrogram signals picked up by electrodes placed in or on the heart and which are coupled by suitable leads to the implantable cardiac rhythm management device. In most devices, the signals emanating from the sense amplifier are applied to one input of a comparator circuit whose other input is connected to a source of reference potential. Only when an electrogram signal from the sense amplifier exceeds the reference potential threshold will it be treated as a detected cardiac depolarization event such as an r-wave or a p-wave. The source reference potential may thus be referred to as a sensing threshold.
In the case of a programmable cardiac rhythm management device the prescribing physician can change the threshold potential of the comparator, but in spite of the flexibility which the programmable threshold offers, malsensing of cardiac depolarization will still occur frequently enough to result in patient discomfort and/or deleterious health effects. This may be due to the fact that cardiac depolarization events (intrinsic beats) can result in widely different peak amplitudes, depending on patient activity body position, drugs being used, etc. Lead movement and noise may further impede the detection of cardiac depolarization events. Noise sources may include environmental noise, such as 60 Hz power line noise, myopotentials from skeletal muscle, motion artifacts, baseline wander and T-waves. When the peak amplitudes associated with cardiac depolarization events become too small relative to a programmed threshold, or when noise levels in the electrocardiogram approach the sensing threshold, the likelihood of oversensing increases (i.e., false detection of depolarization events). If the sensing threshold is increased too high in an attempt to overcome the effects of noise, on the other hand, the likelihood of undersensing (i.e., failing to detect depolarization events) is increased. There is a need, therefore, for methods and apparatus that automatically adjust the sensing thresholds of cardiac rhythm management devices on a continuous beat-to-beat basis in a manner that better avoids both undersensing and oversensing.
The present invention provides a method and apparatus for automatically adjusting the sensing threshold of a cardiac rhythm management device. Such a device may employ both atrial and ventricular sensing channels for sensing atrial and ventricular electrogram signals, where a sensing channel includes a sensing amplifier having one of its inputs connected by a lead to an electrode placed in proximity to either an atrium or a ventricle. The output signal of the sensing amplifier is digitized and passed to a threshold detector that determines whether the amplitude of the signal exceeds a sensing threshold, signifying the detection of either an atrial depolarization event (a p-wave) or a ventricular depolarization event (an r-wave). The device may also include a pulse generator and, associated control circuitry for delivering pacing pulses to the atrium and/or ventricle in response to elapsed time intervals and detected r-waves and p-waves.
In accordance with the invention, atrial and/or ventricular sensing thresholds are automatically adjusted in a manner that attempts to prevent noise signals from being misinterpreted as cardiac depolarization events while at the same time avoiding undersensing of depolarization events that actually occur. In one embodiment, the automatic adjustment is performed by calculating the sensing threshold of a sensing channel based upon a measured amplitude of the depolarization event (i.e., an r-wave or a p-wave) during the current cardiac cycle and a measured current noise level in the channel. A cardiac cycle is defined as the interval between the beginning of one heartbeat and the beginning of another, where the beginning of a heartbeat as defined herein is atrial systole, marked by detection of a p-wave or delivery of an atrial pace, or ventricular systole in the case of a premature ventricular contraction or PVC. The adjustment is preferably performed during the refractory period after either detection of an r-wave or delivery of a ventricular pacing pulse by the device.
In a preferred embodiment, the noise level for a sensing channel is measured during a post-ventricular refractory period. As is conventional, both atrial and ventricular sensing channels are rendered refractory (i.e., where the device ignores detected depolarization events) for a period of time immediately beginning after an r-wave or a ventricular pace. Such a refractory period is referred to as the ventricular refractory period (VRP) for the ventricular channel and the post-ventricular atrial refractory period (PVARP) for the atrial channel. The noise level for a particular channel is measured during a noise measurement window that occurs during the refractory period of the channel, with the sensing threshold being decreased during the noise measurement window for the ventricular channel in order to sense lower amplitude noise activity. The amplitudes of electrogram signals that exceed the decreased sensing threshold during the noise measurement window are measured, and a current noise level is computed based upon the measured amplitudes. The computed current noise level may correspond to, for example, the maximum measured amplitude during the measurement window, an average measured amplitude, or a formula that takes into account the maximum measured amplitude and the noise level calculated for a previous cardiac cycle.
In accordance with the invention, the amplitude of a depolarization event (i.e., a detected r-wave or a p-wave) is measured for the current cardiac cycle and then used along with the current noise level to adjust the sensing threshold for a sensing channel. Preferably, the sensing threshold for a sensing channel is adjusted based upon a current average depolarization event amplitude computed from a combination of a previous average depolarization event amplitude computed for a previous cardiac cycle and, if a depolarization event is detected in the sensing channel for the current cardiac cycle, the current measured amplitude of the depolarization event.
If no depolarization event has been detected in a sensing channel during the current cardiac cycle, it may be surmised that either no such event actually occurred, or that an event occurred but was undersensed. Because of the latter possibility, it would be desirable to adjust the sensing threshold downward (i.e., decrease the threshold) for a sensing channel after a cardiac cycle in which no depolarization event was detected. In accordance with the present invention, therefore, if a heart chamber is paced during a particular cardiac cycle (i.e., because no intrinsic depolarization event was detected), the sensing threshold for that chamber""s sensing channel is adjusted in a manner that decreases the threshold. In certain implementations, the sensing threshold is adjusted so that it is decreased unless the noise level has increased from the previous cardiac cycle to such an extent that the same or a higher sensing threshold is warranted. In a preferred embodiment, this is accomplished by performing the adjustment of the sensing threshold for the channel using an average depolarization event amplitude computed for a previous cardiac cycle that is decreased by a specified constant amount.
As aforesaid, when no depolarization event is detected for a particular sensing channel during a cardiac cycle, the sensing threshold for the channel is adjusted using a decreased average depolarization event amplitude, the effect of which is thus to decrease the threshold as long as the noise level is unchanged from the previous cardiac cycle. In the case where no r-wave is detected and the cardiac rhythm management device is operating in a demand ventricular pacing mode, a ventricular pacing pulse is delivered during the current cardiac cycle. In accordance with the invention, delivery of a ventricular pacing pulse during a cardiac cycle causes the ventricular sensing threshold to be adjusted as described above in a manner that tends to decrease the threshold. In the case where no p-wave is detected during a cardiac cycle and the device is operating in a demand atrial pacing mode, however, there are two possibilities: either an atrial pacing pulse was delivered in response to the non-detection of a p-wave, or an r-wave has been detected that is preceded by neither an atrial pacing pulse nor detection of a p-wave. The latter situation indicates either the occurrence of a premature ventricular contraction or undersensing of a p-wave. In accordance with the invention, non-detection of a p-wave causes the atrial sensing threshold to be adjusted as described above in a manner that tends to decrease the threshold whether the non-detection of the p-wave is due to delivery of an atrial pace or due to a premature ventricular contraction occurring during the current cardiac cycle. In other embodiments, the amount by which the atrial sensing threshold is decreased may be different depending on whether an atrial pace or a premature ventricular contraction occurred during a cardiac cycle by, for example, using separate specified constant amounts by which the average p-wave amplitude computed for a previous cycle is decreased.